Ornithine is converted into arginine in the urea cycle. Intermediaries in the ornithine biosynthesis pathway are important in other steps of this cycle. Amino acid N-acetyl transferase (EC 2.3.1.1) catalyzes the first reaction in a pathway that leads to the synthesis of ornithine from L-glutamate giving N-acetylglutamate as its intermediary product.
Carbamoyl phosphate synthase I, the mitochondrial enzyme that catalyzes the first committed step of the urea cycle, is allosterically activated by N-acetyl glutamate. The rate of urea production by the liver is, in fact, correlated with the N-acetylglutamate concentration. Increased urea synthesis is required when amino acid breakdown rates increase, generating excess nitrogen that must be extracted. Increase in these breakdown rates are signaled by an increase in glutamate concentration through transamination reaction. This situation, in turn, causes an increase in N-acetylglutamate synthesis, stimulating carbamoyl phosphate synthetase and the entire urea cycle.
N-acety lglutamate kinase (EC 2.7.2.8) catalyzes the conversion of N-acetyl-L-glutamate and ATP into N-acetyl-L-glutamate-5-phosphate and ADP. N-acetyl-gamnma-glutamyl-phosphate reductase (EC 1.2.1.38) catalyzes the convertion of N-acetyl-L-glutamate 5-phosphate and NADPH to orthophosphate, NADP and N-acetyl-L-glutamate-5-semialdehyde. This activity is encoded by the argC locus in bacteria and Synechocystis. To date this gene has not been described in plants.
N-2-Acetyl-L-ornithine and L-glutamate are converted to ornithine in the presence of glutamate N-acetyl transferase (EC 2.3.1.35), also called ornithine acetyltransferase. This enzyme is encoded by the argJ locus in bacteria and Synechocystis. This enzyme is active in the mitochondrial matrix as a heterodimer consisting of two subunits processed from the same precursor protein.